# Homework Help: Force on a current currying wire in a magnetic field

1. May 18, 2018 at 8:49 AM

### maxelcat

1. The problem statement, all variables and given/known data
This is a multi choice question from last years aqa a level. We know a positive reading is produced on the scales indicating something is pushing down on the scales.
Please see the picture.

2. Relevant equations
We know current direction, not B field.

3. The attempt at a solution
The positive reading on the scales suggests that the force on the wire is vertically down. Mark scheme says vertically up. We think newton III is needed but not sure how it works.

Can anyone help explain why i is vertically up.

2. May 18, 2018 at 8:59 AM

### Staff: Mentor

Is the wire attached to the scales?

3. May 18, 2018 at 9:00 AM

### maxelcat

nope. = the wire is not attached to the scales, though it is clamped at points D and E.

4. May 18, 2018 at 9:11 AM

### Staff: Mentor

Exactly. The yoke is what is attached to the scales. So rethink things with that in mind.

5. May 18, 2018 at 9:19 AM

### maxelcat

so the yoke is pushed down. Which means the wire is pushing it down. So there must be a downward force on the wire...

6. May 18, 2018 at 9:25 AM

### Staff: Mentor

Yes.

Yes.

No. Consider Newton's 3rd law.

7. May 18, 2018 at 12:52 PM

### maxelcat

well I think you are getting me to say that if the wire is pushing the yoke down then the yoke is pushing it up..... but that doesn't make sense to me because I cant see how the interplay between the current and the mag field works. Suppose I then took away the clamps that are holding the wire DE in place and changed nothing else. The wire would move upwards - yes?

8. May 18, 2018 at 1:05 PM

### Staff: Mentor

Sure. Newton's 3rd law in action. If A pushes B up, then B must push A down.

Yes.